1. Field of the Invention
The present invention relates to coinjection molding shooting pot apparatus and method that cools the shooting pot cylinder, thereby raising the viscosity of the melt therein, thus reducing leakage of the melt between the shooting pot cylinder wall and the shooting pot piston.
2. Description of Related Art
Coinjection molding is typically used to mold multi-layered plastic packaging articles having a laminated wall structure. Each layer is typically passed through a different annular or circular passageway in a single nozzle structure and each layer is partially, sequentially, injected through the same mold gate. Some coinjection hot runner systems include shooting pots to meter material of one plastic resin so that each cavity of a multi-cavity mold receives an accurate dose of that resin in the molding cycle. A shooting pot piston within the shooting pot cylinder is actuated to discharge the resin into a hot runner manifold, which carries the resin to the coinjection nozzle and then into the mold cavity.
Often, a substantial amount of resin drools through the clearance between the shooting pot cylinder and the piston during normal molding operations, when the shooting pot is pressurized and the resin is at high temperature. This drool typically increases with the increase of the piston-to-cylinder wall clearance and with the cylinder temperature. The drool also increases with the decrease of the permanent engagement between the piston and the cylinder wall. Any significant leakage of the resin not only reduces the accuracy of the shot size being injected, but also wastes expensive resin and causes a build-up of degraded resin in the space between the shooting pot cylinder exterior wall and the manifold plate that eventually requires stopping the operation for cleanup. Decreasing the piston-to-cylinder clearance close to a line-to-line fit is not advantageous because of the high risk of the piston seizing inside the cylinder. Also, increasing the piston-to-cylinder engagement in an attempt to minimize drool is often limited by the constraints on the size of the mold.
Attempts have been made to minimize the resin leakage past the shooting pot piston by adjusting the respective sizes of the piston's exterior diameter and the cylinder's interior diameter. A variety of such “sliding fit” tolerance combinations can be tried until the resin leakage is minimized to an acceptable level. However, this fit may have to be readjusted for different resins and for operating the system at different process temperatures where thermal expansion may cause the effectiveness of the “sliding fit” to change. Furthermore this “cut and try” approach is costly as each attempt requires dismantling the hot runner.
U.S. Pat. No. 4,717,324 to Schad discloses a coinjection hot runner assembly with shooting pots. The shooting pot chambers are an integral part of the hot runner manifold that is a heated component of the assembly. The shooting pot pistons that operate within the shooting pots must both slide and seal throughout their operating stroke. There is no disclosure of providing cooling or even lowering the temperature of the piston/shooting pot wall interface to reduce leakage.
U.S. Pat. No. 6,241,932 to Choi shows, in FIG. 1, a conventional two stage (preplasticizing) injection unit including a shooting pot and piston assembly. There is no disclosure of cooling the rear end of the shooting pot to minimize leakage or drooling.
U.S. Pat. No. 6,527,539 to Ujma teaches a shooting pot configuration of an injection molding machine. There is no disclosure of cooling the rear end of the shooting pot to minimize leakage or drooling. See also:                U.S. patent application Ser. No. 10/879,576 entitled INJECTION MOLDING MACHINE SHOOTING POT WITH INTEGRAL CHECK VALVE;        U.S. patent application Ser. No. 10/879,581 entitled INJECTION MOLDING MACHINE SPIGOTTED SHOOTING POT PISTON;        U.S. patent application Ser. No. 10/879,621 entitled APPARATUS AND METHOD FOR SEALING INJECTION UNIT AND SPRUE;        U.S. patent application Ser. No. 10/879,575 entitled APPARATUS AND METHOD FOR ACTUATION OF INJECTION MOLDING SHOOTING POTS;        U.S. patent application Ser. No. 10/879,582 entitled CONTROL SYSTEM FOR A DYNAMIC FEED COINJECTION PROCESS;        U.S. patent application Ser. No. 10/880,494 entitled HOT RUNNER COINJECTION NOZZLE WITH THERMALLY SEPARATED MELT CHANNELS; and        U.S. patent application Ser. No. 10/887,353 entitled APPARATUS AND METHOD FOR INJECTION MOLDING SHOOTING POT WEDGE FEATURE.        
Thus, what is needed is a coinjection molding shooting pot structure which substantially reduces leakage between the shooting pot piston and the shooting pot cylinder wall, and is relatively inexpensive to design, install, and maintain.